1. Field of the Invention
This invention relates to a ball spline having a spline shaft provided with a plurality of raceway grooves, a cylindrical casing moved relatively on the spline shaft and provided with raceway grooves, and balls rolling between the raceway grooves in the spline shaft and those in the cylindrical casing.
2. Description of the Prior Art
There is a known ball spline shown in FIG. 10. This ball spline consists of a spline shaft 50 having support portions at both end parts thereof, and a substantially cylindrical casing 52 moved relatively on the spline shaft 50. The spline shaft 50 is provided with a plurality of raceway grooves 51 in the longitudinal side surfaces thereof. The cylindrical casing 52 is provided with raceway grooves 54 opposed to the raceway grooves 51. Between the raceway grooves 51 in the spline shaft 50 and those 54 in the cylindrical casing 52, raceways along which a plurality of balls 58 roll are formed. The cylindrical casing 52 has a cylindrical casing body 53 provided with return passages 55 and raceway grooves 54, end caps 56 fixed to both ends of the cylindrical casing body 53 and provided with direction switching passages 57, and balls 58 circulatingly rolling along the raceways, direction switching passages 57 and return passages 55. Each of the end caps 56 is provided on its end surface with a side surface seal 59 and a snap ring 60. The cylindrical casing body 53 is provided with a keyway 61 at which the casing body 53 is to be fixed to another part, and an oil hole 62 for a lubricant. Such a ball spline is capable of increasing the cross-sectional area of a spline shaft to the highest possible level, and constitutes a spline bearing in which the cylindrical casing 52 makes an endless linear movement smoothly on the spline shaft 50. A ball spline is applied to the portion of an industrial robot and a transfer machine which has a single shaft and receives a radial load and torque simultaneously.
Japanese Patent Laid-Open No. 152719/1988 discloses a rotation transmission bearing device shown in FIGS. 8 and 9. This rotation transmission bearing device consists of a spline shaft 41 of a substantially circular cross section provided with two raceway grooves 42 in each of two longitudinally extending side surfaces thereof, and a slider 40 having a circulating passage composed of raceway grooves 43 and return passages 44 both of which are formed in a cylindrical inner surface thereof. The slider 40 has amounting surface 49 on which another part 46 is to be mounted, and a plurality of balls 45 incorporated therein rolling in a raceway between the raceway grooves 42 in the spline shaft 41 and the raceway grooves 43. The slider 40 is mounted on the spline shaft 41, and adapted to be moved linearly in the directions of an arrow A as it is guided by the spline shaft 41. A block 48 is set on a bed 47. The spline shaft 41 is supported on the block 48.
Japanese Patent Laid-Open No. 60132/1993 discloses a miniaturized direct-acting rolling guide unit. This miniaturized direct-acting rolling guide unit enables a V-shaped groove to be formed in an upper surface of a base, and a track shaft having a substantially circular cross section to be set easily and accurately in the V-shaped groove, and also makes it possible to easily set a precision measuring instrument and semiconductor equipment on such a base.
Recently, the above-described bearing device or miniaturized direct-acting rolling guide unit has been incorporated into not only an industrial robot and a machine tool but also automobile parts and various kinds of machines, i.e., the range of the use of the bearing device and rolling guide is increasing. Especially, a direct-acting rolling guide unit which enables a track shaft, i.e. a spline shaft to be formed to a smaller diameter has been in increasing demand.
However, regarding the production of a direct-acting rolling guide unit, the productivity of a spline shaft can be increased to a certain extent but a cylindrical casing moved relatively, i.e., moved slidingly on a spline shaft has problems concerning the productivity and manufacturing cost thereof. Namely, since it is necessary that the surfaces of the raceway grooves in a cylindrical casing be smooth and abrasion-resistant, so as to permit the balls to roll smoothly thereon, polishing the inner surface of the cylindrical casing is required, and this causes the productivity of the cylindrical casing to lower and the manufacturing cost thereof to increase. Therefore, in order to manufacture a cylindrical casing for a conventional ball spline, the inner surface of a small diameter of the cylindrical casing has to be polished, and this makes it difficult to produce the cylindrical casing at a low cost and in large quantities.